I am looking for a cross-platform game engine suitable for creating a first person RPG with a continuous playable area of over 65,000 square miles (168,350 square kilometres). In height map terms, that's probably around 131,072 pixels square.

It is my understanding that this would require terrain and object paging/streaming plus some sort of local/relative positioning system to avoid floating point inaccuracies.

Crystal Space has many features that could be useful regarding your needs: a terrain system with paging and splatting, a mesh generator for details such as foliage and rocks, many Level Of Detail systems (static LOD, dynamic LOD with sliding window, and imposters), different water shaders and meshes, a portal system which would be helpful for detailed indoors areas, and a threaded loader for the streamed loading of all of that.

In CEL, there is also a world manager (see the iPcZoneManager property class) which stream load the objects depending on the position of the camera. In CEL, there are also some features such as the path finding and the behavior trees which would help building realistic NPC's.

I don't know how well the positioning problem you mentioned in order to avoid the floating point inaccuracies is managed by the iPcZoneManager property class, but this would at least still to be dealt at the physical simulation level.

If I can work out how maps, regions, zones, sectors, cells and portals relate to each other, I think I may be able to come up with at least one way of doing this.

As a given world space coordinate can belong to more than one sector, I should be able to split the world into small enough pieces that floating point errors never become large enough to cause problems.

Are there any Windows binaries available so that I can test Crystal Space without having to compile the source and install dependencies?

When you want to create a big building or dig a cavern onto the terrain, you would create other sectors and connect them with portals. The sectors would be used to split the big building in small areas that can be loaded separately. Portals are used to go from one sector to another, they are also the main way to create a hole in a terrain (eg for the entrance of a cavern). The default level ('castle') of walktest is a good example of a heavy use of portals (although their exact position should be tweaked).

Is there any other documentation available, such as in a printed book or on another site? I seem to need to jump around the official docs a lot to find all the info on a particular topic.

For example, I read that "Terrain consists of cells, each cell has its own coordinate system (2-axis position and 3-axis scaling)," but couldn't find mention of what coordinate system is used for positioning objects on or above it.

There are several domains where CS is clearly lacking documentation. We would need some help to improve that. For example, if you find the information on the coordinate system of the terrains, then you can fix the documentation and provide a patch (for that, either ask for a programmer account or use the bug tracker http://crystalspace3d.org/trac/CS/report).